As used herein, the term "extensive" or "distributive" mixing means the uniform distribution of particles within a matrix. Distributive mixing can cover the distribution of non-interacting filler particles.
The term "intensive" or "dispersive" mixing refers to the breaking down of gels or agglomerates.
Pelletizing systems generally provide the overall process by which various thermoplastic materials are altered in form, homogenized, mixed, alloyed and combined with additives. As a result, the thermoplastic materials undergo molecular alterations, often referred to as "tailoring". For most products, however, energy input to the resin is kept at a minimum to avoid this effect. Therefore, the pelletizing system must have the ability to control the amount of mixing and/or energy input to the thermoplastic material.
In general, there are two types of pelletizing systems, i.e., non-continuous batch-type and continuous type pelletizing systems.
A pelletizing system that uses a batch-type mixer such as a "Banbury" mixer (as compared to continuous mixers such as the Farrel "FCM") to melt and mix the thermoplastic material and an extruder for melt pressurization is referred to as a two-stage batch-type mixer/extruder pelletizing line.
Such systems have the ability to process a wide range of thermoplastic materials and products that have a wide variation in density, viscosity, etc., but have limited capacities. Further, the atmosphere surrounding the melting/mixing of the thermoplastic material cannot be easily controlled using standard and proven techniques. Therefore, they are of limited value for use in high-capacity pelletizing systems.
In the early stages of development of polymer processing equipment, continuous pelletizing systems had the melting, mixing and pressurization generation function combined in a single machine; this is commonly referred to as a single-stage pelletizing line. An example of such a system is the commonly used, single screw, plasticating extruder. Two-stage extruder/extruder pelletizing systems or tandem extruder pelletizing lines were also developed which included a separate extruder for melting and mixing and another extruder for melt pressurization and additional mixing.
Unfortunately, however, experience has demonstrated that the early continuous pelletizing systems significantly limit the ability to process a wide range of thermoplastic materials and products that have a wide variation in density, viscosity, etc. without extensive modification of the equipment. Typically, the maximum viscosity range that a given extruder can process acceptably is only 75:1 at best, based on melt index (MI). Accordingly, they are not acceptable for use in high-capacity pelletizing systems that must have the ability to process a wide range of thermoplastic materials and products that have a wide variation in density, viscosity etc.
More recently, pelletizing systems have been developed which include a separate mixer and a gear pump for melt pressurization which represent a typical two-stage pelletizing system. Merely as illustrative, U.S. Pat. Nos. 4,032,391 and 4,137,023 assigned to I. Moked et al., relate to a gear pump and a low energy pumping system (LEPSY) and suggest such combination of melter/mixer, such as Farrel continuous mixer (FCM) and such gear pump in an in-line processing system. More recently, according to U.S. Pat. No. 4,452,750 issued to Handwerk et al., there is disclosed an improvement in the operation of a melter/mixer-gear pump system for processing of synthetic thermoplastic materials, the improvement which comprises employing the pressure between the melter/mixer and the gear pump as the controlling parameter which affects, in a proportional relationship, the speed of the gear pump, the energy transmitted to and the consequent temperature of the materials passing through the reciter/mixer.
Although such systems are able to process a viscosity range, based on MI, of approximately 150:1 such systems however are not entirely satisfactory because in certain operations, polymerization reactors are able to produce 300,000:1 or greater viscosity range products. For example, in producing polyethylene in fluidized bed gas phase reactors, polyethylene resin is capable of being produced in large capacities such as in excess of 25,000 lb/hr with a MI range of 300,000:1 or greater.
Moreover, when utilizing conventional technologies for producing large quantities of synthetic thermoplastic resin, it has been found that a significant percentage of the resin is "not mixed" sufficiently as it is processed through the pelletizing system, i.e., the resin "bypasses" the high stress regions within the flow channels of the mixer. Thus, no appreciable dispersive or distributive mixing occurs.
In fact, computational studies have shown that up to 65% of the material can bypass the high stress regions in the mixer. Other studies have shown that a percentage of bypass as low as 5% will reduce film appearance rating (FAR) from +40 to -40. (The more positive the number, the better the rating.)
Attempts to obtain consistent and satisfactory mixing both intensive and extensive with typical, commercially available, twin screw mixers, both tangential counter-rotating and intermeshing co-rotating, have been futile. Extensive testing has also shown that mixers with multiple stages, such as the two stage "LCM" mixer manufactured by Kobe Steel, Ltd., Japan, and multistage long L/D ZSK mixers manufactured by Werner & Pfleiderer, Stuttgart, Germany (W&P), have yielded better, but still not entirely satisfactory, results.
Accordingly, an improved pelletizing system that ensures uniform mixing both intensive and extensive, while preventing and/or controlling "bypass," is required.